If they evolved from prey animals
It sounds like you want a creature with r-selection strategy for survival. "Breeds like rabbits" is a common way to describe it, but you might think more along the lines of "breeds like cicadas." The survival strategy of this species during their evolution towards sapience: to overwhelm their predators with sheer numbers, so that some individuals are certain to survive and reproduce.
Critically, they would have evolved in an environment with abundant resources, such that the main constraint on their population was predator pressure, rather than not having enough to eat (otherwise they would evolve K-selection).
This means that every nearby member of the species is a potential distraction for a predator, but not also a potential rival for food, mates, etc. Neither is cooperation required to obtain these resources, so a fellow person suffering injury or illness doesn't affect your own survival - it actually improves your odds.
Especially if survival conditions are dynamic
There is a short SF story with a species that fits your criteria pretty well: they are very dogmatic and not at all individualistic, to the point that (as the name suggests) they eat their own young, otherwise the population would quickly outstrip available resources, and there is no collective will to figure out a different approach.
A species that evolved r-selection in environments where resource abundance and predator pressure can fluctuate would conform even harder to this herd mentality: we must stick together in times of plenty for safety from our predators, but we must stick together even more in times of scarcity because any individual who defects from the group consensus will endanger the community at-large.
Why do they still act this way?
Now that the aliens have achieved space travel, why do they still act like they did in their prehistory? If you don't want to handwave it as "they evolved this way and it's deeply hard-wired in their biology" you could easily engineer a predator that can still threaten them and encourage this behavior. Something between an antlion and a raccoon could be a good model: a creature capable of infiltrating all but the most secure environments, concealing itself, and striking unexpectedly. Maybe they hibernate for a long time after eating, so that attacks are uncommon but no less frightening - isolation doesn't mean you are safe, only that you're the only morsel in sight when the predator wakes up hungry.